Aggregation Effects on the Emission Spectra and Dynamics of Model

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Aggregation Effects on the Emission Spectra and Dynamics of Model Oligomers of MEH-PPV Gizelle A. Sherwood,† Ryan Cheng,†,‡ Timothy M. Smith,†,§ James H. Werner,| Andrew P. Shreve,| Linda A. Peteanu,†,* and Jurjen Wildeman⊥ Department of Chemistry, Carnegie Mellon UniVersity, 4400 Fifth AVe., Pittsburgh, PennsylVania 15213, Materials Physics and Applications DiVision and Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, and Zernicke Institute for AdVanced Materials, UniVersity of Groningen, Nijenborgh, Netherlands ReceiVed: May 8, 2009; ReVised Manuscript ReceiVed: August 27, 2009

The effects of aggregate formation on the photophysical properties of oligomers of MEH-PPV were studied in bulk solution to better understand the effects of aggregation on the emission properties of the polymer. Nanoaggregates of oligomers from 3 to 17 repeat units in length were formed using a solvent reprecipitation method. The spectra are not readily modeled using the classical dipole-dipole coupling picture of interchain interactions. A strong dependence of the photophysics on the oligomer chain length is also observed. Shortchain oligomers produce nanoaggregates with absorption and emission spectra essentially identical to those of the monomer. Long-chain oligomers form aggregates having more strongly perturbed absorption and fluorescence spectra and decreased emission yields. In these aggregates, the size of the 0-0 band relative to that of the vibronic replicates is a sensitive function of aggregate size and solvent precipitation conditions. Their fluorescence lifetimes are also strongly wavelength dependent. These trends are explained in terms of a core-shell model that postulates the existence of “single-chain-like” and “aggregate-like” emitters within a single aggregate. Introduction

SCHEME 1: Repeat Unit of MEH-PPV Oligomers

Understanding the effects of aggregation on the properties of conjugated materials has emerged as a key factor in optimizing their design and processing for device applications. To date, the primary focus has been on polymeric materials such as MEH-PPV, CN-PPV, and other PPV derivatives, polyfluorene, and thiophenes. As a result, a remarkable amount has been learned at the theoretical and experimental level regarding the effects of aggregation on the spectroscopy, excitedstate dynamics, emission yields, and charge transport characteristics of these materials.1-4 In fact, this body of work has gone a long way toward rationalizing the sometimes contradictory findings on these properties by illuminating how these are affected by details of the synthesis and processing conditions.5-11 Ideally, it will be possible to use this knowledge to maximize the desirable properties of aggregated structures such as good charge transport via strongly interacting π-electron systems, while minimizing the undesirable properties such as emission quenching. It has long been recognized that short-chain oligomers are useful model systems for sorting out the more complex structural † Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213. ‡ Current affiliation: Department of Chemistry, The University of Texas at Austin, 1 University Station, A5300 Austin, Texas 78712. § Current affiliation: Department of Biology, Chemistry and Environmental Science, Christopher Newport University, 1 University Place, Newport News, VA 23606. | Materials Physics and Applications Division and Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos, New Mexico 87545. ⊥ Zernicke Institute for Advanced Materials, University of Groningen, Nijenborgh, Netherlands. * Corresponding author. E-mail: [email protected].

and dynamical properties of the larger polymer species.12-23 In principle, aggregated oligomer chains will exhibit very similar stacking interactions as do chains in the corresponding polymer. However, complexities such as the presence of multiple chain lengths and structural defects24 are expected to be lessened in the oligomer-based systems. In fact, their relative simplicity has made oligomer aggregates a frequent target of electronic structure calculations that seek to explain the spectroscopic properties of the aggregated polymer.4,25-33 It is still an open question as to whether aggregated oligomers exhibit energytransfer dynamics and charge-transport properties similar to the polymers. Recent experiments on energy transfer34 and exciton migration35 in self-organized oligomer stacks suggest these processes can be highly efficient. Unfortunately, there are fewer investigations regarding charge separation36 and transport37 in such species in the literature. In one such study, charge transport was found to be relatively inefficient in chiral stacked oligomer aggregates,38 emphasizing the role of structural order in enhancing this process.37 The current study focuses on alkoxysubstituted oligomer aggregates of MEH-PPV (Scheme 1) formed by reprecipitation from methyl tetrahydrofuran (MeTHF) by methanol (MeOH). By this method, aggregates with a mean size of 200 nm are formed in the case of shorter-chain oligomers (3-7 rings) with longer-chain oligomers having the potential to form larger aggregates under certain solvent reprecipitation conditions.

10.1021/jp904308h CCC: $40.75  2009 American Chemical Society Published on Web 10/05/2009

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Studies of noncovalent assemblies such as aggregates are hindered by the fact that these are formed by relatively weak forces. Therefore, properties such as size and packing geometry can be quite variable and sensitive to details of the preparation. In this regard, the oligomer aggregates are similar to the polymer species. Several earlier studies of short-chain (3-7 repeat units) OPPV aggregates found absorption spectra that were broad and blue-shifted relative to the monomer and emission spectra showing more resolved vibronic structure.12,13,39-41 Recent studies by Janssen et al. and Schenning et al. have focused on elegant ways of narrowing the distribution of structures formed on aggregation by careful choice of substituents that force the aggregates to adopt specific geometries such as helices.18,19 Structural control such as has been achieved in this instance is useful in correlating the spectroscopy and dynamics with theory.42 Novel covalent structures that hold two conjugated molecules in well-defined spatial arrangements have also given very detailed information on the spectroscopic consequences of specific chain-chain interactions.43,44 There has been some very elegant work using mesoporous cavities as a means of controlling aggregation by restricting the number of chains that can enter by varying the cavity pore size.45-47 Likewise, Langmuir-Blodgett techniques,48,49 liquid crystal formation,50,51 and the application of high pressure to polymer films52,53 are other methodologies that have been used to probe the effects of chain-chain interactions in a controlled fashion. Another approach has been to study the effect on polymer spectroscopic properties when it has been deliberately aggregated by using a poor solvent or by preparing a film.8,54-61 For a review of many of the above methods, see ref 62. The current work focuses on alkoxy-substituted oligomer aggregates as a means of reducing the effects of polydispersity and structural heterogeneity expected from the polymer. From an applications standpoint, the most important finding of this study is that these MEH-PPV oligomers can retain their high emission yields upon aggregation. This is in contrast to what has been observed previously in numerous other PPV oligomer aggregates13,18,19,23,63 though with a few exceptions.39 Weak emission, attributed to nonemissive aggregate formation, has also been seen in thin films39,64,65 but again with exceptions.39,66 Specific trends examined in the present study are the effects of aggregation on fluorescence quantum yield as a function of aggregate size and chain lengths of the constituent monomers. For shorter-chain-length oligomers (OPPV5-7), aggregation minimally alters emission yields. Even the longer-chain oligomer aggregates (OPPV9-17), which are better mimics of the polymer, show at most a factor of ∼2 loss in emission compared to that of the corresponding monomers. These differences correlate well with our observations that the emission spectra and fluorescence lifetimes of the short-chain oligomers are essentially unperturbed by aggregation, while more significant changes are seen in these same properties for the long-chain oligomer aggregates. The conclusion that emerges from these and other observations detailed below is that within an aggregate, the strength of the chain-chain interactions increases with increasing oligomer chain length. Therefore, short-chain oligomer aggregates exhibit very “monomer-like” properties, while the properties of long-chain aggregates reflect stronger electronic coupling between the chromophores. Depending on aggregate size and precipitation conditions, we find behavior intermediate between these two extremes is observed. Several explanations for these trends are considered below.

Sherwood et al. Experimental Section Aggregates were prepared via a reprecipitation method in which volumes between 20 and 1000 µL of a 2.9 × 10-6 M stock solution in MeTHF were added to 1 mL of MeOH. The size distributions of the resulting aggregates were determined by dynamic light scattering (DLS) using a Malvern Zetasizer Nanoseries ZS instrument. Whereas, for shorter-chain oligomers (3-7 rings), varying the volume of this MeTHF stock did not affect the size of the aggregates formed, for longer-chain oligomers (9-17 rings), the aggregate sizes were highly sensitive to this parameter. Specifically, increasing the volume of the stock solution used increases the mean aggregate size in a nearly linear fashion. In addition, for the longer-chain oligomers, the sizes and electronic properties of the aggregates are highly dependent on the initial concentration and volume of the stock solution. Therefore, both parameters are specified when describing these spectra. In contrast, neither the mean sizes nor the spectra of the short-chain oligomer aggregates show a strong dependence on formation conditions. Prior and following addition of MeOH, the absorption and emission spectra of the samples were obtained using a Cary 50 UV-visible spectrophotometer and a FluoroMax-2 Jobin Yvon SPEX spectrofluorimeter, respectively. The monomer and aggregate emission spectra were obtained at the absorption maximum of the monomer. Therefore the emission intensity of the aggregate suspension is scaled to account for any differences in the extinction of the aggregate versus the monomer at this wavelength. Aggregates prepared as described above were spin-cast onto a glass coverslip at 10 000 rpm. Prior to deposition, the coverslips were treated by briefly passing them through the flame of a butane torch to remove surface impurities. This protocol yielded well-separated aggregates with little or no emission from the substrate background. To obtain corresponding images of the monomer, the 2.9 × 10-6 M stock solution of each oligomer in MeTHF was spin-cast as described above. In all cases, this yielded a uniform film rather than aggregated structures. As this film photobleached, the remaining material were more dispersed diffraction-limited bright structures that showed the “on-off” intensity fluctuations or blinking indicative of single isolated chromophores.67 The aggregate and film samples were excited using the 454 nm output of an argon ion laser (BeamLok 2060, SpectraPhysics) with a power of 1 mW at the back of the objective. They were imaged onto a CCD detector (Cool Snap HQ) using an inverted IX-71 Olympus microscope in through-objective total internal reflection (TIRF) mode (Olympus PlanApo 60X oil immersion objective, N.A. ) 1.45). A 495 nm long pass filter was used to diminish background due to scattered light. To obtain fluorescent lifetimes, we excited the monomer solution and aggregate suspension with a 437 nm pulsed diode laser (LDH-PC-440, PicoQuant GmbH), and the decays were recorded using time correlated single photon counting electronics (SPC-630, Becker and Hickl or PicoHarp300, Picoquant). The emission was detected using a cooled MCP-PMT (Hamamatsu) coupled to a monochromator (Acton Spectra Pro 150) or a single photon avalanche photodiode (MPD) with filters used for wavelength selection (Chroma and Thorlabs). Results Small Aggregates. Because short-chain (3-7 rings) and longchain (9-17 rings) oligomers show dramatically different behaviors on aggregation via the reprecipitation methods used here, we will discuss these separately below.

Dynamics of Model Oligomers of MEH-PPV

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Figure 1. (a) DLS histogram of OPPV7 aggregates. (b) Absorption and emission spectra of OPPV7 in its monomer (red) and aggregate (blue) form. Excitation wavelength is 445 nm.

Figure 2. Emission spectra of an unfiltered solution of OPPV7 (blue) and solution after filtration with 200 nm pore filter (red). Difference in these two spectra (black) represents the emission due to aggregates removed by filtration (see text).

Bulk solution-phase data for a representative short-chain oligomer, OPPV7, are shown in Figure 1. The DLS histogram (Figure 1a) shows a relatively narrow and single-peaked aggregate size distribution for OPPV7 (200 ( 40 nm). Whereas strongly blue-shifted absorption spectra and perturbed emission spectra were observed previously for aggregates of several other 3-5 ring PPV oligomer aggregates,13,39,40 here we find that the absorption and emission spectra of OPPV7 in its monomeric and aggregated form are nearly identical (Figure 1b), with only a small change being seen in the relative intensities of the two vibronic bands. This intriguing find suggests that aggregation does not ineVitably lead to emission quenching in OPPV-type molecules, at least for relatiVely short-chain lengths. This same behavior is seen in OPPV3 and OPPV5 under all of the aggregation conditions we have explored using MeTHF and MeOH as the solvent pair (data not shown). As noted earlier, several shorter-chain oligomer aggregates have also been previously shown to have emission yields in aggregated or film forms that are similar to the monomeric form.39,66 In order to evaluate the possible contribution of free monomers to the emission spectra of the shorter-chain oligomers in MeTHF/MeOH, we obtained the emission spectra of these solutions before and after filtering to remove aggregates 200 nm and larger. The results of this comparison for OPPV7 (Figure 2) indicate that ∼86% of the fluorescence signal is recovered in the eluant following filtration and that the residual emission is identical to that of the monomer. Because the DLS results unequivocally demonstrate the formation of aggregates in the mixed solvent system, one interpretation of this result is that

Figure 3. TIRF microscopy images of OPPV7: (a) monomeric form, (b) aggregated form. In both samples, the concentration of OPPV 7 is ∼10-6 M, but in the monomeric form, the sample is cast from MeTHF, while in the aggregated form the sample is cast from a mixture of MeTHF and MeOH (see text).

the aggregates formed are very weakly emissive relative to the monomer, while a second possibility is that aggregates of shorter-chain oligomers are weakly bound and are readily resolubilized in the filtration step. In addition, because the DLS reports that the mean size of the aggregates is 200 ( 40 nm, the 200 nm filter used would only eliminate the largest aggregates in this distribution. To help address these potential ambiguities, we turned to fluorescence microscopy, which has the advantage of allowing us to readily differentiate between monomeric and aggregated molecules (Figure 3). Spin-casting a 10-6 M solution of OPPV7 in MeTHF yields a thin film with no signs of aggregate formation, while spin-casting a 10-6 M solution in MeTHF/MeOH reveals isolated aggregates that are similar in size to that reported by DLS.68 Similar results are seen for OPPV5 (data not shown). Clearly a large number of the aggregates are highly emissive, indicating that they would be expected to contribute substantially to the bulk solution phase spectra as well. Therefore, we conclude that filtration disrupts the initially formed aggregates, causing them to pass through the filter as monomers or as smaller aggregates that scatter too weakly to be measured by DLS. Longer-Chain Oligomer Aggregates. As noted earlier, the longer-chain OPPV aggregates (9-17 rings) exhibit varied absorption and emission behavior that is highly sensitive to aggregate size and to the details of the reprecipitation protocol used. As expected, this behavior is much more similar to that of the polymer, MEH-PPV. This comparison will be considered in detail below. A clear illustration of the sensitivity of the aggregate spectra of longer-chain oligomers to aggregate size is seen in OPPV13. A comparison between the absorption and emission spectra for two extremes of aggregate size, ∼200 nm (small) and ∼ 1.5

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Figure 4. (a) DLS of OPPV13 small aggregate preparation (b) and corresponding absorption and emission spectra of the monomer (red) and small aggregate (blue) samples. (c) DLS of OPPV13 large aggregate preparation (d) and corresponding absorption and emission spectra of the monomer (red) and large aggregate (blue) samples. Excitation wavelength is 460 nm.

µm (large), is shown in Figure 4, along with the corresponding spectra of the monomer and DLS histograms of each sample. The absorption spectrum of the small aggregates (Figure 4b) exhibits a weak shoulder at ∼510 nm that is red-shifted ∼2500 cm-1 with respect to the peak of the monomer absorption at 460 nm. Moreover, the overall absorption spectrum is diminished in intensity by a factor of ∼2 relative to that of the monomer. This loss in intensity is reproducible and is seen even though both solutions were prepared from the same starting monomer concentration and have been corrected for dilution effects (Experimental Section). This effect has precedent in the literature,12 and it may arise here because of the high effective optical density of the aggregate, although the bulk optical density of the suspension is relatively low. In contrast, the absorption spectrum of the larger aggregates (Figure 4d) is more similar in appearance to that of the monomer and more similar to it in integrated intensity. The emission spectra of the monomer and the small and large aggregates of OPPV13 differ primarily in the intensity of the 0-0 band relative to the other vibronic features. Specifically, the small aggregates show dramatically reduced intensity in this band relative to either the larger aggregate or the monomer. There is, in fact, a systematic decrease in the 0-0 band intensity with decreased aggregate size (Figure 5). This loss of intensity at the high-energy edge of the emission spectrum does not appear to be due to self-absorption within the aggregate as the effect becomes more pronounced as the aggregate size decreases, which is opposite of what is expected for selfabsorption. When corrected for concentration and changes in the extinction coefficient at the excitation wavelength, all aggregate emission spectra have nearly the same intensity, which is roughly half that of the OPPV13 monomer. This loss of emission intensity on aggregation is unique to the longer-chain oligomer and is consistent with the shortening of the fluores-

Figure 5. Evolution of the emission spectra of OPPV13 with varying aggregate sizes. Mean sizes of 1500 nm (magenta), 900 nm (dark blue), 700 nm (green), 500 nm (red), and 200 nm (black) are shown. Spectra are scaled to account for variations in the extinction values between aggregates of different sizes at the excitation wavelength (460 nm). Arrow indicates the trend in the 0-0 band emission intensity with decreasing aggregate size (see text).

cence lifetime seen on aggregation of OPPV13 described below. Figure 6 contains fluorescence microscope images of a film made from depositing the monomer in MeTHF on a microscope slide at ∼10-6 M concentration (Figure 6a) as compared to small aggregate (Figure 6b) and large aggregate (Figure 6c) preparations. Note that the trend in size distributions of the aggregates formed from the two different ratios of MeTHF and MeOH is consistent with that obtained from DLS. To quantify the possible contribution of residual monomer to these aggregate spectra, we again compared the emission of the aggregate solutions before and after filtration with a 200 nm pore filter. Whereas suspensions containing predominantly small aggregates show negligible free monomers in the eluant



Figure 6. TIRF microscopy images of OPPV13: (a) monomer, (b) small aggregate, and (c) large aggregate preparations. In all three samples, the concentration of OPPV13 is ∼10-6 M, but in the monomeric form the sample is cast from MeTHF, while in the aggregated forms the samples are cast from a mixture of MeTHF and MeOH (see text).

Figure 7. Effect of filtration on the emission spectra of OPPV13: (a) small aggregate (800 nm) suspensions. Spectra of the suspensions prior to filtration (blue) and after filtration (red) are shown. The difference in these spectra (black) represents the emission due to aggregates removed by filtration. (c) Comparison of the difference spectra for the large aggregate (dash) and small aggregate (solid) preparations. The spectra are arbitrarily scaled to 1.

(Figure 7a), those containing primarily large aggregates exhibit a noticeably higher monomer contribution in the eluant (32%) (Figure 7b). In both cases, the emission spectrum of the filter eluant was subtracted from that of the unfiltered suspension to give what we term the aggregate residual spectrum. Comparison of the two aggregate residual spectra (Figure 7c) indicates that the large aggregate residual spectrum is more monomer-like than is the small aggregate residual spectrum with regard to the relative intensities of its 0-0 and 0-1 bands. Bulk solution-phase fluorescence lifetime measurements using TCSPC on each monomer and aggregate system corroborate many of the trends described above. Aggregates of the shortchain oligomers OPPV5-7 have lifetimes identical to those of the corresponding monomers, consistent with their similar fluorescence intensities (data not shown). In all cases, high quality fits to the fluorescence decays were obtained with two exponential functions in which the shorter lifetime component comprises over 95% of the decay amplitude. The emission dynamics are independent of collection wavelength, and the results are insensitive to the fitting method used. The lifetimes obtained for OPPV5 monomer and aggregates are both 0.8 ns, while those for OPPV7 are both 0.7 ns. Turning to the long-chain oligomers, specifically OPPV13, once again the monomer exhibits predominantly single exponential decays that are wavelength independent (Table 1) However, very different fluorescence decay behavior is seen for the small and large aggregate preparations of these oligomers. The wavelengths selected in Table 1 correspond to the maxima of the monomer and aggregate spectra (Figures 4, 5, and 8) of OPPV13. As noted earlier, a common feature of these long-chain aggregates is the evolution of their emission spectrum with aggregate size (Figures 5 and 8). In all three cases, the high-energy band of the large aggregate is coincident in wavelength with the corresponding monomer origin and loses intensity relative to the other vibronic peaks as aggregate size decreases. Focusing on OPPV13 aggregates (Table 1), it is evident that the smaller aggregates (A3-A5, Figure 8) have

similar emission lifetimes across their spectral bandwidth with the shortest lifetime component dominating the decay. The emission decays of a representative small aggregate (A4, Figure 9) as a function of wavelength show that the slope of the initial fast component is similar at all three wavelengths studied but that the relative contribution of slower decay components increases as the collection wavelength moves to the red. The results in Table 1 suggest that this initial fast decay has two components, one at ∼0.25 ns and one at ∼ 0.8 ns. Similar behavior is seen for smaller aggregates of OPPV9 and OPPV17 (not shown). In each case, the fastest decay component is roughly a factor of 2-3 shorter than the corresponding monomer decay, also shown in Figure 9. This is consistent with the decrease in emission yield that is seen on aggregation and indicates that the loss in fluorescence intensity is due to the appearance of a new nonradiative decay channel or enhancement of the rate of an existing channel. In terms of the effects of aggregation on fluorescence lifetime, the oligomer aggregates behave similarly to MEH-PPV itself.69 In contrast, the decay dynamics of the larger aggregates vary significantly with collection wavelength, indicating a higher degree of spectral heterogeneity than in the small aggregates. The following trends, seen in emission dynamics of OPPV13, are common to all three longer oligomer chain lengths (OPPV9-17, data not shown). Figure 10 shows the decays of the two largest aggregates of OPPV13 (A1 and A2) as a function of emission wavelength. The fact that the decay kinetics at 510 nm are essentially identical to that of the monomer is eVidence that this band arises either from residual free monomer in solution or from “monomer-like” chains present in the aggregate. A similar picture is suggested by the results of the aggregate filtration experiments described above. In contrast, each of the longer-wavelength kinetics is dominated by a noticeably faster component, which is very similar to that seen in the smaller aggregates. This is evident in Figure 11, which compares the decays of all five aggregate sizes obtained at collection wavelengths of ∼580 nm. It also appears that the

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TABLE 1: Fluorescence Lifetimes of OPPV13 Monomer and Aggregates as a Function of Emission Wavelengtha mean aggregate size (nm)

510 nm

537 nm

577 nm

monomer A1b

>1000

0.59 (>99) 0.57 (98) 1.4 (2)

0.59 (>99) 0.45 (89)c 1.0 (11)

A2

∼800

A3

∼600

A4

∼400

A5

∼200

0.43 (62.8) 0.77 (37) 3.5 (0.2) 0.26 (57) 0.85 (42) 3.3 (1) 0.25 (48)e 0.79 (50) 2.4 (2) 0.24 (81)e 0.82 (15) 2.7 (4)

0.33 (79) 0.79 (20) 3.2 (1) 0.23 (85) 0.92 (14) 3.3 (1) 0.33 (84) 1.0 (14) 3.6 (2) 0.24 (89) 0.94 (9) 3.1 (2)

0.30 (80)d 0.71 (19.7) 3.3 (0.3) 0.30 (87) 0.86 (12) 3.3 (1) 0.24 (78) 1.0 (20) 3.5 (2) 0.32 (87) 1.2 (11) 3.8 (2) 0.27 (88) 1.1 (10) 3.5 (2)

630 nm 0.3 (81) 0.99 (17) 4.1 (2) 0.29 (83) 1.3 (14) 4.3 (3) 0.29 (83) 1.3 (14) 4.3 (3) 0.28 (80) 1.4 (16) 3.8 (4)

a Monomer lifetimes are fit to a single exponential function, while aggregate decays are fit to a triple exponential function. The approximate percent contribution of each lifetime component is in parentheses. b A1 through A5 refer to distinct preparations that produce the range of aggregate sizes shown. Corresponding emission spectra are shown in Figure 8. c Three exponential fits of this decay gives time constants of 0.39 ns, 0.73 ns, and 3.9 ns (Supporting Information). d Collection wavelength was 586 nm for aggregate A1. e Collection wavelength was 520 nm for aggregates A4 and A5. Fits and residuals obtained for the OPPV13 monomer and for each collection wavelength of aggregate A1 using both 2 and 3 exponential functions are given in the Supporting Information.

Figure 8. Normalized emission spectra of various OPPV13 preparations used in fluorescence lifetime measurements as labeled in Table 1: A1 (black), A2 (red), A3 (blue), A4 (green), and A5 (magenta).

contribution of a slower decay component increases as aggregate size decreases. This may be indicative of some population of excimers, particularly in the smaller aggregates. It is interesting to note that the trends in fluorescence lifetime with emission wavelength observed in these aggregates are opposite to those seen in the polymer and in aggregates of numerous other conjugated oligomers. In MEH-PPV films, for example, the decay of the emission from the blue edge of the fluorescence spectrum exhibits a prominent fast component that is absent in the decay curves of the lower-energy emission.70-72 This has been interpreted to indicate rapid energy transfer from higher-energy chain segments in the polymer to lower-energy or aggregated chains. The fact that the aggregates studied here do not appear to exhibit this trend suggests either that energy transfer is relatively inefficient in these systems or that the transfer rate is significantly faster than the current time resolution of our system and therefore does not contribute to the observed decay dynamics. More experiments are planned to address this issue. In summary, the oligomer aggregates formed by reprecipitation in MeTHF/MeOH are highly emissive and exhibit fairly narrow spectral features despite the broad size distributions that result from this method. Aggregation has little or no effect on

Figure 9. Fluorescence decays (O), fits (solid lines), and IRF (dashed line) for OPPV13 monomer (red) and small aggregate A4 collected at 537 nm (blue), 577 nm (green), and 630 nm (magenta). Each wavelength has a bandpass of (3-5 nm, and the samples are in fluid solution. Emission spectrum of aggregate A4 is shown in Figure 8 and the parameters from the fits are summarized in Table 1. Decays are scaled for comparison.

the emission spectra and fluorescence lifetimes for the shorterchain oligomer aggregates, at least for the solvent reprecipitation conditions used here. Aggregation induces more significant spectral perturbations for the longer-chain species and causes the fluorescence lifetimes to decrease by roughly a factor of 2. In the Discussion below, we will interpret our results in terms of existing theories regarding the effect of aggregation on the electronic properties of conjugated molecules. Discussion Mechanisms by which aggregation can perturb the spectra of individual models basically separate into two classes: interchain and intrachain models. These two extremes are defined by the relative contributions of intrachain (exciton) and interchain (excimer and polaron pair) interactions to the overall



Figure 10. Wavelength dependence of fluorescence decay kinetics in large aggregates A1 (left) and A2 (right) of OPPV13 in fluid solution. Both panels contain the decay of the OPPV13 monomer (red O), the fit (red solid line), and the IRF (dashed line). The emission decays (O) and fits (solid lines) collected at 510 and 586 nm are shown for aggregate A1 and those collected at 510 nm (blue), 537 nm (green), and 577 nm (magenta) are shown for aggregate A2. Aggregate emission spectra are shown in Figure 8, and the parameters from the fits are summarized in Table 1. Decays are scaled for comparison. Fits and residuals obtained for the OPPV13 monomer and for each collection wavelength of aggregate A1 using both 2 and 3 exponential functions are given in the Supporting Information.

Figure 11. Dependence of the emission dynamics on aggregate size. Data (O) and fits (solid lines) for aggregate A1 (blue), A2 (green), A3 (black), A4 (red), and A5 (magenta) are shown for emission collected at 577-586 nm for samples in fluid solution. Aggregate emission spectra are shown in Figure 8, and the parameters from the fits are summarized in Table 1. Decays are scaled for comparison.

wave function of the system and have been discussed extensively intheliteratureofconjugatedoligomersandpolymers.6,9,23,25,28,30,65,73,74 Excimer-like interactions generally cause relatively unstructured and red-shifted emission due to the unbound nature of the ground state potential energy surface. Excimers also have lifetimes significantly longer than the radiative lifetime of the monomer because of the forbidden nature and therefore weak oscillator strength of the transition between the excimer state and ground state. Such interactions tend to prevail in molecules which favor cofacial stacking and those having strongly electronegative substituents such as cyano-PPVs and cyano- and fluorine-substituted oligomers.6,14,22,23,75 Whereas excimer formation implies little or no charge transfer between chains, transfer of a full charge between chains leads to the formation of interchain polaron pairs. Such species are likely to be more longlived in conjugated polymers than oligomers because the opportunity for charge to migrate significantly along the polymer chain before recombination can occur.76 Nonetheless, charged carriers have been reported as short-lived transients in the ultrafast spectroscopy of oligomer nanocrystals at energies ∼0.9 eV above the bandgap.36 As is the case for many other alkyl or alkoxy-substituted PPV oligomer aggregates in the literature, there is little evidence for

excimer or interchain emission in the aggregates studied here, except perhaps for the smaller OPPV13 aggregates (see above).22 However, there has been controversy in the literature as to whether the absence of excimer emission necessarily precludes the presence of a dark excimer-like state that decays predominantly by nonradiative channels.6,65,73 Such a state may act as a nonradiative pathway to depopulate the state initially accessed by optical excitation, if the rate of this relaxation process is competitive with the fluorescence lifetime. Though this mechanism may explain the reduction in emission lifetime seen in some aggregates as discussed below, it would not explain the spectral properties of the aggregates described here. Next, we turn to another class of models that we believe most accurately represents our data. Frequently, the dual emission and/or anomalous Franck-Condon envelopes that are often seen in aggregates of conjugated polymers are rationalized in terms of what is sometimes known as “two-state” pictures.60,61,77-82 In these descriptions, the polymer aggregate consists of regions that are “single-chain like” or blue emitting and regions that are aggregated and emit to the red. Because, for PPV-like molecules, the reported shifts between monomer and aggregate maxima are ∼0.1-0.2 eV and the vibronic widths are ∼ 0.07 eV,83 the spectra of the two species in fact overlap substantially. In most such models described to date, variations in the degree of overlap as well as in the relative populations of the two species and the efficiency of energy transfer between them all conspire to produce anomalous vibronic envelopes in the emission spectra of aggregates of conjugated molecules such as those described above. Understandably, this model makes no specific predictions regarding the relative intensities of the vibronic peaks of the aggregate spectrum but instead suggests that these will vary from aggregate to aggregate as has been seen by Kim et al. in single-molecule emission spectra of MEHPPV.82 An alternate version of the “two-state” model would be one in which the two emitters are regions of the aggregate that have a high degree of packing symmetry and those that are more disordered. The “two-emitter” models often invoke variations in energy transfer efficiency between aggregates as being an important source of variations in their emission spectra,61,77-79,81,82 though this does not appear to be a critical element of the model. As noted earlier, we have no evidence at this point that energy transfer is efficient in the aggregates studied here. We belieVe that our aggregates consist of “monomer-like” or weakly associated oligomers and “aggregate-like” or more densely packed oligomers with the relatiVe amounts of each

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depending on Variables such as chain length and precipitation conditions. For example, in solution, OPPV5 and OPPV7 aggregates are predominantly “single-chain like” as their absorption and emission spectra and lifetimes are identical to those of the corresponding monomeric species. This is evident when examining aggregate filtration experiments in which 90% of the emission profile is recovered after filtration. The weakness in the ground state interchain interactions in short-chain oligomer aggregates likely reflects the fact that dispersion forces increase and solubilities of the chains decrease with increased chain length. It may also reflect the influence of substituents in disrupting the close packing that is generally observed in the unsubstituted chains or simply a change in preferred packing arrangements with chain length.14,23,84 It is notable that aggregates formed by these oligomers in other solvent systems such as tetrahydrofuran and water actually do exhibit spectral shifts relative to the monomer (unpublished data). This suggests that the aggregate packing structure and/or interchain interaction strength are strong functions of the solvent-solute interactions that induce aggregation. These effects will be elaborated in a subsequent contribution from our group. For longer-chain aggregates, such as OPPV13, monomer-like and aggregate-like regions coexist in the solvent suspension with the latter being dominant in the smaller aggregates. This heterogeneity is consistent with the observation of monomerlike fluorescent lifetimes for the emission band that coincides with the monomer origin (∼510 nm) and a shorter component to the emission decay for the lower-energy bands. The fact that aggregation perturbs the emission properties of the longer-chain aggregates more than it does the shorter-chain aggregates would at first glance appear inconsistent with theoretical work that predicts a weakening in the expected interchain electronic coupling for increased chain length.25,32,85 This effect would be counterbalanced if on average the ground state chain-chain interactions were to be stronger in the long-chain aggregates than in the short-chain aggregates. There are several reasons to believe this might be the case. One is the expected increase in the interchain dispersion interactions with increasing chain length.86 This would favor tighter packing in the long-chain aggregates.87 The second is the decrease in solubility of the chains with increasing chain length. This suggests that as the chain length increases, chain-chain interactions become on average stronger than chain-solvent interactions such that coalescence into the aggregated state becomes more favorable. On the basis of the above considerations, we believe that the packing motif for the OPPV aggregates in solution suspension can be described as a “core-shell structure”, meaning that a central closely packed core is surrounded by less densely packed chains that are in contact with the surrounding solVent.81 In this picture, the less densely packed chains give rise to the primarily monomer-like emission peaked at 510 nm, while the more closely packed chains give rise to aggregate emission at the redder wavelengths. It is interesting to note that contrary to the polymer, there does not appear to be energy transfer between the blue- and red-emitting chains, at least at the time resolution of the current experiment. Having outlined the reasons for favoring a core-shell or dual emitter model to explain the emission spectroscopy and dynamics of the OPPV aggregates, we will briefly mention several other models in the literature to explain the spectroscopy of molecular aggregates. Assessing the applicability of specific theories to the aggregates described here is made challenging by the dearth of structural data and likelihood that they are disordered. However, our justification for attempting such a

Sherwood et al. comparison is the following. By and large, models are used to infer packing structure (which is difficult or impossible to measure in many aggregated systems) from spectroscopic data, which is relatively easy to obtain rather than the reverse. In this spirit, we are not attempting to validate these models, which can only be done by comparing their predictions to findings on structurally well-characterized systems. Rather, we are treating them as alternatives to the core-shell model described above to explain several specific phenomena observed in our data. These are the variation in the relative intensities of the 0-0 band and vibronic replicates in OPPV13 aggregates (Figures 4, 5, and 8) with the aggregate size and apparent increase in the perturbations to the electronic spectra and the emission yields and decay dynamics with increased oligomer chain length. The classic work in the field that describes how the spectrum of an aggregate is perturbed relative to that of the monomer is attributed to Kasha and to Davydov.88,89 Briefly, in aggregates formed from strongly interacting monomers, the monomer absorption will be split into two bands, one corresponding to the in-phase and one to the out-of-phase coupling of the monomer transition moments. Transitions from the ground state to these two states will be allowed or forbidden, respectively. If the allowed state is the lower of the two in energy, its absorption will be red-shifted relative to that of the monomer, and the aggregate will be strongly emissive (J-aggregates). Alternatively, if the allowed state is the higher-energy state, the aggregate will absorb at higher energies than the monomer, and it will be weakly emissive (H-aggregates). The energy ordering of these two states in turn depends on the packing geometry of the chromophores. Considering the simplest case of a dimer, we find the critical parameter is the slip angle between the two transition moments. As this angle decreases from 90° (card-pack structure) to ∼55° (brickwork structure), the spectral properties of the dimer evolve from those characteristic of an H-aggregate to those of a J-aggregate. This basic exciton coupling model has been used to rationalize the weak emission and blue-shifted absorption spectra that have been observed in several PPV oligomer aggregates.18,20,22,41,63,90 These spectral perturbations are likewise seen in thiophene aggregates because of their relatively large Davydov splittings, at least for short chain lengths.29,91-93 However, in this form, this model does not explain the spectroscopy of the OPPVn aggregates described here. Specifically, none of the aggregate absorption spectra reported here show clear evidence of Davydov splitting or a shift to higher energy due to H-aggregate formation; in fact, the perturbations we observe in the absorption spectra are very modest. Moreover, even in aggregates for which the emission is weaker than that of the monomer (OPPV9-17), the absorption and emission spectra exhibit small red shifts rather than the blue shifts that would be predicted by a simple exciton coupling model. In the past few years, a few authors have considerably refined the model described above.4,26,33,41 These models in part attempt to explain the apparently anomalous vibronic profiles in the dominant CdC progression seen in the emission of numerous small molecule and polymer aggregates.4,31,93,94 Both predict that, for certain molecular packing arrangements, the 0-0 band will be strictly forbidden, though the vibronic replicates will be observed, leading to an apparent shift of the emission spectrum by one vibrational quantum in the CdC stretch. Disorder in the packing is expected to lead to a breakdown of this selection rule such that there is some emission in the 0-0 region. The effect is predicted to be an anomalous Franck-Condon envelope for the disordered aggregates.94 We note that changes in the

Dynamics of Model Oligomers of MEH-PPV vibronic progression of the OPPV13 aggregate solution-phase spectrum with aggregate size (Figure 5) could be consistent with either of these models, though the observed variation of fluorescence lifetime with vibrational quantum number would not necessarily be predicted. Moreover, the aggregates studied here are not expected to adopt the molecular packing arrangements that are associated with these effects.14,23,84 The other models we will mention here focus on the evolution of the spectral properties of oligomer aggregates with chain length as well as packing geometry and interchain separation.25,95 Two interaction regimes were identified. At interchain distances greater than the size of the individual oligomer, the electronic wave functions of each component of the dimer remains localized, and the interactions between the two components are governed by dipole-dipole terms as described by Kasha and Davydov.88,89 As the chains pack more closely, the wave function becomes increasingly delocalized over the two chains, and the splitting between the allowed and forbidden excited state levels dramatically increases. These authors found that the splitting decreases with increased chain length or more precisely increased conjugation length and increases with decreased chain-chain separation. Likewise, the critical chain-chain separation, below which the wave functions are delocalized, roughly corresponds to the oligomer length.25 This behavior is consistent with a breakdown of the point-dipole approximation that occurs when the delocalization length of each chain is longer than the distance between chains.32,85 Our observation that the perturbations induced in the fluorescence spectra and dynamics upon aggregation increase with increasing oligomer chain length run counter to these predictions. However, as described below, we can rationalize this discrepancy by noting that a number of factors, including chain-chain interaction strength, may also change with chain length, making simple one-parameter correlations difficult to extract from the data. In summary, though the emission spectra of the OPPV aggregates presented here are surprisingly structured, uniform, and show clear trends with chain length, the details of the spectral perturbations seen upon aggregation are very sensitive to packing and solvent conditions, which complicates comparisons with existing theories. The emission spectra of the aggregates show characteristics of monomer-like and aggregatelike phases, depending on chain length, whereas such behavior has not been previously reported for either thiophene or PPV oligomer aggregates. This may derive from the fact that, at least in the crystal phase, substituted oligomers are known to favor different packing arrangements than the herringbone packing found in unsubstituted oligomers.14,23,84 Thus far the discussion has focused primarily on the fluorescence spectroscopy of the OPPV aggregates. Here we turn briefly to consider what more is learned from the TCSPC experiments. In general, aggregates having diminished transition moments to the ground state exhibit fluorescence lifetimes considerably longer than the radiative lifetime of the monomer as a consequence. This trend is clearly observed in several cyano-PPV films, which form excimers and show lifetimes of ∼5 ns as compared to the estimated radiative lifetime of ∼1.7 ns.6,96 While the longer-chain aggregates show a 2-4 ns component to their emission decays, which could be excimerlike in origin, this represents 20% or less of the overall decay amplitude and contributes even less to the emission decay of the shorter-chain oligomer aggregates. The aggregates studied here, therefore, do not emit strongly from an excimer- or exciplex-like state. This is not, however, inconsistent with exciplex formation as long as the decay from the exciplex state

J. Phys. Chem. C, Vol. 113, No. 43, 2009 18859 to the ground state is predominantly nonradiative.73 However, the dominant aggregate emission lifetime component is either identical to that of the monomer (OPPV5 and OPPV7) or shorter than that of the monomer (OPPV9-17). Earlier, we had attributed the insensitivity of the fluorescence lifetime to aggregation in shorter-chain aggregates to weak chain-chain interactions. Here, we will focus on possible mechanisms for the decrease in fluorescence lifetime in the longer-chain oligomer aggregates. In general, ignoring possible changes in nonradiative rates, the fluorescence lifetime of a molecule or molecular assembly can decrease if its radiative lifetime decreases or in other words if there is an increase in oscillator strength to the ground state. Whereas it is possible for the oscillator strength to increase on aggregation, this is not apparent experimentally (compare monomer and aggregate absorption spectra in Figure 4). Alternatively, if it is assumed that the absorbing and emitting states are the same, the fluorescence lifetime may decrease, if the rates of the competing nonradiative processes increase. This would in turn decrease the fluorescence quantum yield. The longer-chain aggregates show diminished fluorescence yields experimentally (compare monomer and aggregate emission spectra in Figure 4). In fact, there is fairly good consistency between the factor by which the yield is decreased on aggregation (2-3) and the factor by which the rate of the dominant fluorescence lifetime component decreases upon aggregation. This is consistent with an increase in the nonradiative rate upon aggregation being the dominant effect that we observe. While it is difficult to prove the identity of a particular nonradiative channel in a molecule or aggregate, we here consider several possibilities from the literature of conjugated systems. Because MEH-PPV shows similar effects on the fluorescence lifetime and yield due to aggregation,96-98 it is should be particularly useful to examine those mechanisms that have been invoked for the polymer. Therefore, one may gain insight into the nature of the nonradiative decay channel in the aggregates by reference to the extensive literature on this subject in MEH-PPV and related polymers. Time-resolved fluorescence measurements of MEH-PPV in solution generally agree on a single exponential fluorescence lifetime of ∼330 ps.6,72,96,99 The radiative lifetime, however, is estimated to be ∼0.8-1 ns,100 consistent with the fluorescence quantum yield being in the range 0.25-0.35.64 The specific relaxation mechanisms that cause the solution-phase oligomer and polymer fluorescence quantum yields to be less than unity are not, however, unequivocally established in the literature. As the singlet-triplet crossing is known to be inefficient in PPV oligomers12 and in MEH-PPV,101 other nonradiative processes are likely. The shared feature of these mechanisms as applied to polymer chains is that they require the presence of a lowenergy dark state (weak or zero transition moment to the ground state) to which the initial excitation can transfer rapidly enough to diminish the emission lifetime and lead to a reduced fluorescence yield. One that is thought to be important in dilute polymer chains is the presence of defect sites either in the carbon chain or due to the presence of carbonyl groups leading to intrachain nonluminescent acceptor states that are populated via rapid energy transfer.24,100,102-106 This mechanism seems unlikely to explain the drop in fluorescence yields in the longer-chain aggregates as it is not apparent why these chain defects should be increased simply upon aggregation or why they are more likely to be present in long-chain versus short-chain oligomer aggregates. Moreover, our current data does not appear to support rapid energy transfer among chains in the solution-phase

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aggregates. This would be required if a small number of defects were to substantially alter the emission yields of the aggregate samples. In the polymer literature, it has been shown that weakly emitting low-energy interchain excitons or excimers may also provide an efficient nonradiative channel that can be populated by rapid energy migration through the chain.6,65 It is certainly possible that such species are formed in the longer-chain aggregates. For example, the ∼4 ns tail in the emission decay seen at longer wavelengths in the OPPV13 oligomer aggregates may have some contribution from an excimer-like species as this lifetime is longer than the ∼0.8 ns radiative lifetime of the OPPV13 monomer. Measurements of the steady-state emission spectra and fluorescence lifetimes of the aggregates at low temperatures are planned to identify possible interchain states as emission from these states is thought to increase with decreasing temperature. Other interchain dark states can arise due to H-aggregate formation as described earlier. In a polymer containing both isolated chains and aggregated regions, rapid energy transfer to the dark exciton level of the aggregate would cause a reduction in yield. In the case of isolated aggregates, rapid internal conversion to this level quenches the fluorescence. Finally, a dark state can arise due to dissociation of the exciton into free charges. This is expected to be more facile in the polymer than in oligomer aggregates, but further experiments are planned to test this assumption. One factor that argues against many of these processes as being the cause of reduced emission yield in the long-chain aggregates is the fact that the solution-phase aggregates formed here do not appear to undergo rapid energy transfer, unlike MEHPPV itself (vide infra). Therefore, even if a low energy nonluminescent trap state exists in these aggregates, it is not clear that the rate of energy transfer to this state would be rapid enough to cause the observed reduction in fluorescence lifetime. Therefore, we expect that any trap state that diminishes the yield of the oligomer aggregates must be present on a significant number of the constitutent chains. Both Tretiak et al. and Cornil et al. have predicted the existence of low-energy dark states in aggregates, which could be the cause of the increased nonradiative decay in the aggregates relative to the monomers.25,28 Another mechanism to consider is that the rate of internal conversion (direct relaxation from S1 to S0) may be enhanced by aggregation. Internal conversion is the predominant nonradiative channel in the isolated chains, and the efficiency of this process increases with increasing chain length, tracking the decreaseinthefluorescenceyieldandfluorescencelifetimes.12,13,18,39,107 Electrofluorescence measurements have shown that the fluorescence yield of low-temperature isolated OPPV oligomer and of MEH-PPV is decreased in an applied electric field, establishing that the rate of internal conversion in the isolated chains can be sensitive to environmental factors such as polarity and polarizability of their environment.108 It is therefore possible that the local dielectric environment within the aggregate may alter the relative energies of the S0 and S1 states enough to increase the rate of internal conversion and therefore decrease the emission yield. Electrofluorescence measurements on oligomer aggregates are planned that will allow this mechanism to be tested further. Summary and Conclusions Aggregation effects on the emission spectroscopy and yields in MEH-PPV were examined using a model system of aggregated OPPVn oligomers formed by a solvent reprecipitation method. This method yields relatively large aggregates (>100 nm) that nonetheless have quite uniform spectral properties as

Sherwood et al. judged by their well-resolved emission spectra. Essentially, no emission quenching is observed for the shorter-chain oligomer aggregates (OPPV5-7), and their absorption and emission spectra and fluorescence lifetimes are likewise unperturbed. These observations are consistent with weak chain-chain interactions in these aggregates. In contrast, ∼50% quenching is observed in the emission of aggregates of the longer-chain oligomers (OPPV9-13). Moreover, these aggregates show significant changes in their vibronic spectra with aggregate size, primarily in the intensity of the 0-0 band relative to that of the vibronic replicates. Wavelength-dependent emission decays are also observed. We find these properties to be most consistent with the formation of a “core-shell” structure in which there are areas of the aggregate core that are “monomer-like” and contain weakly interacting chains and those that are “aggregatelike” and contain strongly interacting chains. These results illustrate the importance of using longer-chain oligomer aggregates to model polymer behavior. Acknowledgment. L.A.P. acknowledges a Special Creativity Extension to NSF CHE-0109761 and NSF CHE-079112 for financial support. Acknowledgment is also made to the Donors of the American Chemical Society, Petroleum Research Fund for support of this research (PRF 44317-AC4). This work was performed in part at the Center for Integrated Nanotechnologies, a U.S. Department of Energy, Office of Basic Energy Sciences user facility at Los Alamos National Laboratory (Contract DEAC52-06NA25396) and Sandia National Laboratories (Contract DE-AC04-94AL85000). We also thank Dr. David Yaron (CMU) and Dr. Frank Spano (Temple University) for helpful discussions. Supporting Information Available: Fits and residuals for OPPV13 monomer and OPPV13 aggregate A1 at all collection wavelengths are given as well as comparisons between the results obtained by fitting by 2 versus 3 exponentials in terms of the quality of the fit and the parameters obtained. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Schwartz, B. J. Annu. ReV. Phys. Chem. 2003, 54, 141. (2) Bredas, J.-L.; Beljonne, D.; Coropceanu, V.; Cornil, J. Chem. ReV. 2004, 104, 4971. (3) Barbara, P. F.; Gesquiere, A. J.; Park, S.-J.; Lee, Y. J. Acc. Chem. Res. 2005, 38, 602. (4) Spano, F. C. Annu. ReV. Phys. Chem. 2006, 57, 217. (5) Yan, M.; Rothberg, L. J.; Kwock, E. W.; Miller, T. M. Phys. ReV. Lett. 1995, 75, 1992. (6) Samuel, I. D. W.; Rumbles, G.; Collison, C. J. Phys. ReV. B 1995, 52, R11573. (7) Jakubiak, R.; Collison, C. J.; Wan, W. C.; Rothberg, L. J.; Hsieh, B. R. J. Phys. Chem. A 1999, 103, 2394. (8) Hsu, J. H.; Fann, W. S.; Tsao, P.-H.; Chuang, K.-R.; Chen, S.-A. J. Phys. Chem. A 1999, 103, 2375. (9) Nguyen, T.-Q.; Doan, V.; Schwartz, B. J. J. Chem. Phys. 1999, 110, 4068. (10) Nguyen, T.-Q.; Kwong, R. C.; Thompson, M. E.; Schwartz, B. J. App. Phys. Lett. 2000, 76, 2454. (11) Piok, T.; Gadermaier, C.; Wenzl, F. P.; Patil, S.; Montenegro, R.; Landfester, K.; Lanzani, G.; Cerullo, G.; Scherf, U.; List, E. J. W. Chem. Phys. Lett. 2004, 389, 7. (12) Oelkrug, D.; Egelhaaf, H.-J.; Gierschner, J.; Tompert, A. Synth. Met. 1996, 76, 249. (13) Oelkrug, D.; Tompert, A.; Gierschner, J.; Engelhaaf, H.-J.; Hanack, M.; Hohloch, M.; Steinhuber, E. J. Phys. Chem. B 1998, 102, 1902. (14) van Hutten, P. F.; Krasnikov, V. V.; Hadziioannou, G. Acc. Chem. Res. 1999, 32, 257. (15) Bredas, J. L.; Cornil, J.; Beljonne, D.; Dos Santos, D.; Donizetti, A. A.; Shuai, Z. Acc. Chem. Res. 1999, 32, 267. (16) Bredas, J. L.; Beljonne, D.; Cornil, J.; Calbert, J. P.; Shuai, Z.; Silbey, R. Synth. Met. 2001, 125, 107.

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JP904308H

Aggregation Effects on the Emission Spectra and Dynamics of Model

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